[RE-wrenches] NEC 705.12(B)(2)(3)(c) intention

Fri Apr 28 07:45:53 PDT 2017

Ah, I better understand your question now August. You are basically
pointing out that in any loadcenter, if the total sum of all loads can be
limited then why would we limited the sum of all sources that feed that
panelboard? I've certainly heard this questioned before and I understand
where it is coming from. The challenge with this argument is that it really
applies to all types of connections between multiple sources and loads,
including commercial/industrial and residential and any source such as
utility, solar, wind, rotating generators and even storage. In order to
modify any NEC language for the situation you mention, all other types of
interconnections would need to be considered. In general, there must be a
single OCP to prevent conductors or busbars from overload. It is a rare
case in the Code where a bus is allowed to be protected through the use of
multiple smaller OCPs that sum together. Our PV combiner boxes can be one
of those rare cases. The old split-bus service panel would be another.
Within standard AC loadcenters it is a particularly problematic argument
since additional breakers can be added or increased in size very easily. A
simple warning label may not be adequate by many people to protect the bus
from overload conditions. Because of that the Code has allowed us some room
to size the sum of all sources higher than the bus rating (the 120% rule),
and perhaps there is a good argument to allow even more room, but I think
it would be hard to say that you could feed a 200A loadcenter with two 200A
sources (which is how others will view your example) without creating some
solid argument for how the bus is protected from overload. Perhaps with
smarter panelboards in the future containing other protections such as
thermal protection, load control, source control, etc. we would have the
substantiation needed to pursue this type of design but until we have those
products, standards, and new Code allowances (which probably will come as
Microgrids develop) we are most likely stuck with the relatively
straightforward requirements found in 705.12. Which I would say have
substantially improved in NEC 2014 and 2017.

If you are motivated I would suggest you take a stab at writing up some new
language that you like and to think through how an argument would be made
to support it. We have entered the NEC2020 cycle and public inputs are due
in September so you do have a window to get your ideas into the
discussions. If you are not aware of it SEIA is helping to coordinate an
effort as part of the group we call "the PV Industry Forum" to develop
broad consensus proposals for the next edition. If you want to join this
effort send an email to NEC2020 at seia.org or let me know.

Cheers,

Jason Fisher
NEC CMP-4 Principal Rep for SEIA

On Apr 20, 2017 6:45 AM, "August Goers" <august at luminalt.com> wrote:

Hi Jason and All –

Thanks for the responses. I think it is fair to say that the 200 A of loads
plus the 200 A of solar (albeit through a feedthrough lug) equals 400 A as
Jason mentions below.

I guess my real question is whether limiting the loads to 200 A plus
allowing up to 200 A of solar (using my 200 A meter/main example) is a safe
setup even if it is not currently NEC compliant. Perhaps future editions of
the NEC could be expanded to allow this? Or is there a reason that NEC
705.12(B)(2)(3)(c) is written the way it is and we must include the PV too?

Cheers,

August

*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
Behalf Of *Jason Fisher (STC)
*Sent:* Thursday, April 20, 2017 6:10 AM

*To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
*Subject:* Re: [RE-wrenches] NEC 705.12(B)(2)(3)(c) intention

Hi August.

If the main panel had a 400A rated bus and the sub was 200A then I think
you could make a good argument but it appears the main is a 200A bus so
therefore the sum of all load and supply breakers on that bus is 400A
(loads plus solar excluding the main breaker), which does not comply with
the Code language nor intent. The feed through lugs do not create a single
bus, you now just have two buses and a feeder conductor to evaluate. Once
the solar is connected you there are two sources feeding the main panel
bus, not just the utility. You can exclude one but not both when evaluating
the main panel bus.

Hope I correctly assessed your situation.

Jason Fisher

On Apr 19, 2017 6:01 PM, "Mark Frye" <markf at berkeleysolar.com> wrote:

True, but, the intent of the Code is not to always allow the
interconnection of the maximum amount of PV power possible, so long as an
over-current event cannot occur.

Here we see Code that prevents over-current events in all cases. I guess
you could say that that is it's intent.

Sadly, the case you show would not be one of those allowed.

Happily, many that would not have been allowed in the past are now allowed.

This is called progress.

On 4/19/2017 4:31 PM, August Goers wrote:

Right, but if we limit the load breakers to the bus rating per NEC
705.12(B)(2)(3)(c), then there will never be an overcurrent event.

August

*From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
Behalf Of *Mark Frye
*Sent:* Wednesday, April 19, 2017 4:18 PM
*To:* RE-wrenches <re-wrenches at lists.re-wrenches.org>
*Subject:* Re: [RE-wrenches] NEC 705.12(B)(2)(3)(c) intention

If your sketch shows that there are 200A of breakers in the sub panel and
200A of breakers in the main, then, no, that would not be the intent.  The
lug kit makes the two separate bars into a single bar.

Mark

On 4/19/2017 3:29 PM, August Goers wrote:

Hi Wrenches –

Now that we’re in the 2014 NEC, we’ve increasingly been using the NEC
705.12(B)(2)(3)(c) interconnection method:

[image:
imap://markf@mail.lmi.net:143/fetch%3EUID%3E.Re-Wrenches%3E12200?header=quotebody&part=1.1.2&filename=image001.png]

This is a great way of putting lots of PV on a panel when there are few
loads. As long as the total breaker rating (excluding the supply breaker)
equals the bus rating or less, we’re good to go. However, we recently ran
into a panel that had a lug subfeed kit installed in it.

Do you think the intention of the code is to allow something like shown in
this sketch?

Cheers,

August

Luminalt

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